There's several tutorials available on YouTube which I found helpful.
I started off reading tutorials online and subscribing to photoshop magazine (when I was a web designer). Using it day in day out for 5 years helped too.
Start with some online tutorials and get stuck in. Cloning or simple image manipulation (cropping, contrast, saturation, curves, levels, sharpening etc) is probably a first point of call?
There's threads on TP that might be worth a look too in the post processing forum.